Re: [PATCH v9 Kernel 3/5] vfio iommu: Add ioctl defination to unmap IOVA and return dirty bitmap

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 15 Nov 2019 00:26:26 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 11/14/2019 1:52 AM, Alex Williamson wrote:
> > On Thu, 14 Nov 2019 01:22:39 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >   
> >> On 11/13/2019 4:00 AM, Alex Williamson wrote:  
> >>> On Tue, 12 Nov 2019 22:33:38 +0530
> >>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >>>      
> >>>> With vIOMMU, during pre-copy phase of migration, while CPUs are still
> >>>> running, IO virtual address unmap can happen while device still keeping
> >>>> reference of guest pfns. Those pages should be reported as dirty before
> >>>> unmap, so that VFIO user space application can copy content of those pages
> >>>> from source to destination.
> >>>>
> >>>> IOCTL defination added here add bitmap pointer, size and flag. If flag  
> >>>
> >>> definition, adds
> >>>      
> >>>> VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP is set and bitmap memory is allocated
> >>>> and bitmap_size of set, then ioctl will create bitmap of pinned pages and  
> >>>
> >>> s/of/is/
> >>>      
> >>>> then unmap those.
> >>>>
> >>>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> >>>> Reviewed-by: Neo Jia <cjia@nvidia.com>
> >>>> ---
> >>>>    include/uapi/linux/vfio.h | 33 +++++++++++++++++++++++++++++++++
> >>>>    1 file changed, 33 insertions(+)
> >>>>
> >>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >>>> index 6fd3822aa610..72fd297baf52 100644
> >>>> --- a/include/uapi/linux/vfio.h
> >>>> +++ b/include/uapi/linux/vfio.h
> >>>> @@ -925,6 +925,39 @@ struct vfio_iommu_type1_dirty_bitmap {
> >>>>    
> >>>>    #define VFIO_IOMMU_GET_DIRTY_BITMAP             _IO(VFIO_TYPE, VFIO_BASE + 17)
> >>>>    
> >>>> +/**
> >>>> + * VFIO_IOMMU_UNMAP_DMA_GET_BITMAP - _IOWR(VFIO_TYPE, VFIO_BASE + 18,
> >>>> + *				      struct vfio_iommu_type1_dma_unmap_bitmap)
> >>>> + *
> >>>> + * Unmap IO virtual addresses using the provided struct
> >>>> + * vfio_iommu_type1_dma_unmap_bitmap.  Caller sets argsz.
> >>>> + * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP should be set to get dirty bitmap
> >>>> + * before unmapping IO virtual addresses. If this flag is not set, only IO
> >>>> + * virtual address are unmapped without creating pinned pages bitmap, that
> >>>> + * is, behave same as VFIO_IOMMU_UNMAP_DMA ioctl.
> >>>> + * User should allocate memory to get bitmap and should set size of allocated
> >>>> + * memory in bitmap_size field. One bit in bitmap is used to represent per page
> >>>> + * consecutively starting from iova offset. Bit set indicates page at that
> >>>> + * offset from iova is dirty.
> >>>> + * The actual unmapped size is returned in the size field and bitmap of pages
> >>>> + * in the range of unmapped size is returned in bitmap if flag
> >>>> + * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP is set.
> >>>> + *
> >>>> + * No guarantee is made to the user that arbitrary unmaps of iova or size
> >>>> + * different from those used in the original mapping call will succeed.
> >>>> + */
> >>>> +struct vfio_iommu_type1_dma_unmap_bitmap {
> >>>> +	__u32        argsz;
> >>>> +	__u32        flags;
> >>>> +#define VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP (1 << 0)
> >>>> +	__u64        iova;                        /* IO virtual address */
> >>>> +	__u64        size;                        /* Size of mapping (bytes) */
> >>>> +	__u64        bitmap_size;                 /* in bytes */
> >>>> +	void __user *bitmap;                      /* one bit per page */
> >>>> +};
> >>>> +
> >>>> +#define VFIO_IOMMU_UNMAP_DMA_GET_BITMAP _IO(VFIO_TYPE, VFIO_BASE + 18)
> >>>> +  
> >>>
> >>> Why not extend VFIO_IOMMU_UNMAP_DMA to support this rather than add an
> >>> ioctl that duplicates the functionality and extends it??  
> >>
> >> We do want old userspace applications to work with new kernel and
> >> vice-versa, right?
> >>
> >> If I try to change existing VFIO_IOMMU_UNMAP_DMA ioctl structure, say if
> >> add 'bitmap_size' and 'bitmap' after 'size', with below code in old
> >> kernel, old kernel & new userspace will work.
> >>
> >>           minsz = offsetofend(struct vfio_iommu_type1_dma_unmap, size);
> >>
> >>           if (copy_from_user(&unmap, (void __user *)arg, minsz))
> >>                   return -EFAULT;
> >>
> >>           if (unmap.argsz < minsz || unmap.flags)
> >>                   return -EINVAL;
> >>
> >>
> >> With new kernel it would change to:
> >>           minsz = offsetofend(struct vfio_iommu_type1_dma_unmap, bitmap);  
> > 
> > No, the minimum structure size still ends at size, we interpret flags
> > and argsz to learn if the user understands those fields and optionally
> > include them.  Therefore old userspace on new kernel continues to work.
> >   
> >>           if (copy_from_user(&unmap, (void __user *)arg, minsz))
> >>                   return -EFAULT;
> >>
> >>           if (unmap.argsz < minsz || unmap.flags)
> >>                   return -EINVAL;
> >>
> >> Then old userspace app will fail because unmap.argsz < minsz and might
> >> be copy_from_user would cause seg fault because userspace sdk doesn't
> >> contain new member variables.
> >> We can't change the sequence to keep 'size' as last member, because then
> >> new userspace app on old kernel will interpret it wrong.  
> > 
> > If we have new userspace on old kernel, that userspace needs to be able
> > to learn that this feature exists (new flag in the
> > vfio_iommu_type1_info struct as suggested below) and only make use of it
> > when available.  This is why the old kernel checks argsz against minsz.
> > So long as the user passes something at least minsz in size, we have
> > compatibility.  The old kernel doesn't understand the GET_DIRTY_BITMAP
> > flag and will return an error if the user attempts to use it.  Thanks,
> >   
> 
> Ok. So then VFIO_IOMMU_UNMAP_DMA_GET_BITMAP ioctl is not needed. I'll do 
> the change. Again bitmap will be created considering smallest page size 
> of iova_pgsizes
> 
> But VFIO_IOMMU_GET_DIRTY_BITMAP ioctl will still required, right?

Yes, I'm not willing to suggest a flag on an unmap ioctl that
eliminates the unmap just so we can re-use it for retrieving a dirty
page bitmap.  That'd be ugly.  Thanks,

Alex